1. Field of the Invention
The present invention relates to an actinic-ray- or radiation-sensitive resin composition employed in a semiconductor production process for an IC or the like, a circuit board production process for a liquid crystal, a thermal head or the like and other photoapplication lithography processes, and also relates to a method of forming a pattern with the use of the composition. More particularly, the present invention relates to an actinic-ray- or radiation-sensitive resin composition that is suitable for exposure by means of a liquid-immersion projection exposure unit using far-ultraviolet rays of wavelength 300 nm or shorter as a light source, and also relates to a method of forming a pattern with the use of the composition.
In the present invention, the terms “actinic rays” and “radiation” mean, for example, a mercury lamp bright line spectrum, far-ultraviolet rays represented by an excimer laser, extreme ultraviolet rays, X-rays, electron beams and the like. In the present invention, the term “light” means actinic rays or radiation.
2. Description of the Related Art
In accordance with the miniaturization of semiconductor elements, the wavelength shortening of the exposure light source and the realization of high numerical apertures (high NA) for projector lenses have been advanced. At present, an exposure machine of 0.84 NA using an ArF excimer laser of wavelength 193 nm as a light source has been developed. As is commonly known, the following formulae can be established therefor.(Resolving power)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 
In the formulae, λ is the wavelength of the exposure light source; NA is the numerical aperture of the projector lens; and k1 and k2 are factors relating to the process.
As a technology for enhancing the resolving power of an optical microscope in order to attain a further resolving power enhancement by wavelength shortening, it is heretofore known to employ a liquid immersion technique, that is, a method in which a space between a projector lens and a sample is filled with a liquid of high refractive index (hereinafter also referred to as a “liquid for liquid immersion”).
The “effect of the liquid immersion” is as follows. Taking λ0 as the wavelength of exposure light in air, n as the refractive index of the liquid for liquid immersion to air and θ as the convergent half angle of the light beam, where NA0=sin θ, the above-mentioned resolving power and focal depth in the event of liquid immersion can be expressed by the following formulae.(Resolving power)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
That is, the effect of the liquid immersion is equivalent to the use of an exposure wavelength of 1/n. In other words, in projection optic systems of identical NA, the liquid immersion would enable the focal depth to be n-fold. This is effective in all pattern configurations. Further, this can be combined with a super-resolution technology, such as a phase shift method or a modified illumination method, now under study.
Examples of the apparatuses utilizing this effect in the transfer of the microscopic image pattern of a semiconductor element are introduced in patent references 1, 2, etc.
The recent progress of the liquid immersion exposure technology is reported in non-patent references 1, 2, etc. In the use of an ArF excimer laser as a light source, it is presumed that pure water (refractive index at 193 nm: 1.44) can offer most promising prospects as the liquid for liquid immersion from the viewpoint of handling safety as well as 193-nm transmission and refractive index.
Since the emergence of the resist for a KrF excimer laser (248 nm), an image forming method through chemical amplification has been employed as a resist image forming method in order to compensate for any sensitivity deterioration caused by light absorption. Brief description of an image forming method through positive chemical amplification is given below by way of example. Upon exposure, an acid generator will be decomposed at exposed areas to thereby generate an acid. In baking after the exposure (post-exposure bake [PEB]), the generated acid is used as a reaction catalyst so that an alkali-insoluble group is converted to an alkali-soluble group. Thereafter, alkali development is carried out to thereby remove the exposed areas. Thus, the relevant image forming method is provided.
With respect to the transfer of a pattern, when the pattern is microscopic, the width of the line obtained by development may vary due to the coverage of an exposure mask even if the pattern size of the exposure mask is unchanged. The possible impact thereof on productivity is a matter of concern (see patent reference 7 below).
The resist for an ArF excimer laser (193 nm) utilizing this chemical amplification mechanism is now mainstream. However, it is pointed out that when such chemical amplification resist is applied to liquid immersion exposure, as the resist layer is brought into contact with the liquid for liquid immersion during the exposure, not only would the resist layer suffer a property alteration but also components having an unfavorable influence on the liquid for liquid immersion would leach from the resist layer. Patent reference 3 describes an instance of resist performance alteration caused by immersing the resist for ArF exposure in water before and after the exposure, and in the reference this is noted as being a problem in the liquid immersion exposure. Patent reference 4 describes an instance of suppressing the above-mentioned leaching by the addition of a siliconized or fluorinated resin.
Moreover with respect to the liquid immersion exposure process, in the event of exposure using a scan type liquid immersion exposure machine, when the liquid for liquid immersion fails to move while tracking a moving lens, the exposure scan speed must be lowered. This negatively affects productivity. When the liquid for liquid immersion is water, it is preferred for the resist film to be hydrophobic from the viewpoint of superiority in water tracking properties.
In recent years, increasing the exposure scan speed of an exposure machine is attempted in view of the production efficiency. Accordingly, hydrophobizing the resist film to a higher degree is needed. When the water tracking property is unsatisfactory, water drops remain on the resist film upon the exposure scan and thus would remain as defects after development (watermark defects) to thereby cause a yield decrease. In contrast, it is known that when the hydrophobicity of the resist film is extremely high, air trapping occurs along the scanning direction, which leads to a change of refractive index, causing image formation to be unsuccessful and defects generally known as bubble defects to occur, a further factor of decreased yield. With respect to techniques for enhancing the water tracking property, patent reference 5 and patent reference 6 describe resins having a norbornane skeleton.
Apart from the defects peculiar to the liquid immersion operation, it is known that hydrophobizing the resist film lowers the water wettability during the rinse operation after development, thereby increasing the amount of development residue, another factor leading to decreased yield.